1. Field of the Invention:
The present invention relates to a process for preparing a high molecular weight polyphosphonate.
2. Description of the Prior Art:
The polyphosphonates are marked by their high flame retardance and thermal stability. In addition they are often used to impart flame retardance to various polymers.
Heretofore, the following processes have been used to prepare the polyphosphonates:
1. The arylphosphonic dihalide and the aromatic diol are charged into a reactor and are heated with stirring in an inert gas atmosphere while both the rise in temperature and decrease in pressure are controlled according to the progress of the dehydrohalide polycondensation reaction, whereby a molten polyphosphonate is given. (melt-polycondensation process).
2. The arylphosphonic dihalide and the aromatic diol and a suitable solvent are charged in a reactor and are heated with stirring to effect a dehydrogen-halide (i.e. removal of hydrogen halide) polycondensation reaction, whereby a polyphosphonate solution is given. (solution polycondensation process).
3. The arylphosphonic dihalide is dissolved in a water insoluble solvent and the solution is added with stirring to an alkaline aqueous solution of the aromatic diol to effect the polycondensation reaction. A polyphosphonate solution is obtained by separating the water phase. (interfacial polycondensation process).
In these conventional processes, the polycondensation reaction has been improved by using a catalyst, if desirable.
In the first process, no solvent is required and, accordingly, the process is simple. However, in order to perform the polycondensation reaction, it is necessary to raise the reaction temperature to a point higher than the melting point of the polyphosphonates. Moreover, the handling of the polyphosphonates is troublesome because the viscosity of the reaction system increases during the progress of the polycondensation reaction, reaching several hundred thousand centipoise or higher. Also, because hydrogenhalide is generated by the polycondensation, a serious corrosion problem occurs and a special corrosion resistant reactor is necessitated. Moreover, since the polycondensation is performed at high temperature for a long time, the resulting polyphosphonate can be colored by side reactions or contaminated by insoluble by-products, deteriorating the product quality.
On the other hand, in accordance with the processes (2) and (3) the polycondensation reaction can be performed under relatively mild conditions. Consequently, it is possible to employ a reactor having only a glass lining on its inner surface. The difficulty of selecting non-corrosive material for the reactor is not involved and high quality products having no coloring can be obtained easily. However, in these processes it is necessary to separate the polyphosphonate from the reactant solution. The ordinary practice, particularly (2), process 92), is to pour the solution into a large amount of non-solvent in order to precipitate the polyphosphonate, and follow with subsequent separation and drying. However, the following disadvantages are attendant to this technique:
1. In order to perform the precipitation smoothly a relatively large amount of reaction solvent, e.g., more than two times the amount of the polyphosphonate, is required. PA1 2. In order to precipitate the polyphosphonate, more than several times as much non-solvent as solvent is required. PA1 3. The time required for the precipitation is relatively long. PA1 4. In order to remove the solvent from the precipitated solid, many washings with the non-solvent are required. PA1 5. The precipitated solid is usually bulky. PA1 6. The solid separated from the solution contains a large amount of the non-solvent and, consequently, it takes a long time to dry. PA1 7. From the large volume of the mixture of the non-solvent and the solvent, each component must be separated. PA1 a. A high concentrated solution of the polyphosphonate can be used. PA1 b. No non-solvents are required. PA1 c. The solidification can be efficiently performed in a short time. PA1 d. The drying is quite easy.
The separation and recovory of the product and the solvent are quite complicated, whereby a large sized apparatus is required and the process has a low efficiency. The polyphosphonate is relatively unstable and easily decomposed by hydrolysis or alcoholysis. In the separation and recovery processes, the complicated treatment last for a long time whereby the hydrolysis of the polyphosphonate is made quite possible. When an alcohol is used as the non-solvent, alcoholysis of the polyphosphonate can occur instead, thereby decreasing the quality of the product, especially the molecular weight.
There is another process for separating and recovering the polyphosphonate from the solution in which the solution is heated to evaporate the solvent. In this process, the viscosity of the solution increases as the concentration rises, the heat-conductance of the mixture is low and the handling of the materials is quite difficult. Accordingly, it is not amenable to being performed on an industrial scale. Moreover, the product is colored and insoluble by-products are produced. The quality of the product also suffers because the highly viscous solution of the polyphosphonate is treated at high temperature for a long time.
It would be highly desirable to have a new process for separating the polyphosphonate which is not subject to the above disadvantages.